In the prior art, a higher executive standard, for example, including peak rates of 1 Gbps in the downlink and 500 Mbps in the uplink is defined for a Long Term Evolution-Advanced (LTE-A) system as compared to a Long Term Evolution (LTE) system. Also good compatibility of the LTE-A system with the LTE system is required. Carrier Aggregation (CA) is introduced to the LTE-A system so as to meet the requirements including peak rates improvement, compatibility with the LTE system and full use of spectrum resources.
In CA, an eNB can configure a User Equipment (UE) concurrently with a plurality of cells and enable the plurality of cells to provide the UE concurrently with a data transmission service. In the CA-enabled LTE-A system, carriers corresponding to the respective cells may be consecutive or inconsecutive in the frequency domain, the maximum bandwidth of each carrier is 20 MHz for compatibility with the LTE system, and there is a bandwidth which may be the same or different across the respective carriers.
In the LTE system, there are defined access mechanisms, and random access takes place generally for the following several reasons:
There is an initial access including an access from a Radio Resource Control Idle (RRC_IDLE) status (also referred to an initial access) and Radio Resource Control (RRC) connection reestablishment initiated after a radio link fails;
Random access is required for handover procedure;
A UE in a Radio Resource Control Connected (RRC_CONNECTED) status has downlink data arrival; and
A UE in an RRC_CONNECTED status has uplink data arrival.
In the two scenarios with downlink data arrival and a handover, if there is a dedicated preamble, then the UE can perform a contention-free random access, and if there is no dedicated preamble, then the UE can perform a contention random access.
In order to facilitate transmission of uplink data and to send Hybrid Automatic Repeat Request (HARD) feedback information for downlink data, the UE shall maintain uplink synchronization with the eNB. As specified in the LTE system, uplink synchronization with the UE is maintained at the network side. Referring to FIG. 1, when the UE performs random access, the eNB at the network side obtains an initial Timing Advance (TA) from the preamble, the eNB and the UE maintain a Time Alignment Timer (TA Timer or TAT) respectively, the eNB sends a TA command to the UE and starts the TAT, and if the UE can not receive the TA command correctly, then the UE sends a Negative Acknowledgement (NACK) message to the eNB, and the eNB resends a TA command and restarts the TAT a period of time T1 after receiving the NAK, of if the UE receives the TA command correctly, then the UE starts (restarts) the TAT of the UE and sends an Acknowledgement (ACK) message to the eNB, and the eNB restarts the TAT of the eNB upon reception of the ACK message sent from the UE and resents a TA command after a period of time T2, where the eNB considers a specific UE as being synchronized if its TAT for the UE does not expire; and the UE also considers itself as being synchronized as long as the TAT maintained by the UE does not expire.
In the LTE-A system, due to the introduce of CA, the UE has to initiate random access procedure for a plurality of cells in numerous scenarios so as to enable uplink synchronization of the respective cells, and in order for the UE to maintain TAs of the respective cells, the concept of TA group has been introduced where the same TA can be used for granted carriers of cells belonging to the same TA group and there are different TAs for granted carriers of cells belong to different TA groups. The eNB will send well planned TA groups to the UE so that the UE can know which of cells configured for the UE by the eNB belong to the same TA group.
In the prior art, in the LTE-A system, among the plurality of cells configured for the UE, typically one of the cells is a primary cell (pcell) and the other cells are secondary cells (scells). Reference can be made to the implementation schemes of the LTE system to perform random access procedure for the primary cell (pcell), but no relevant procedure has been defined regarding how to initiate random access procedure for the secondary cells (scells) to maintain uplink synchronization with the network side at the respective cells, so further improvements are desirable.